Does he get stressed out by the water changes? You could do a change every other day, rather than ever day. He might just be stressed by all the water changes.

The 'salt smell' isn't anything to worry about. You're adding salt, so that water smells a little salty.

Here's what I'd try:
1) Change water every other day. (Rather than every day.)
2) Put a towel or sheet around the tank. You don't need to cover the tank completely, but cover it at least partially. Being in darkened conditions reduces stress and has a calming effect.
3) Feed him one of whatever his favorite food is. If he doesn't eat it, remove it and try again later.

Does he have any symptoms or issues other than not eating, and the back end sinking?